Imperial Mountain Dew Taster
I think you might be reading "party arcanist" a bit narrowly. I took it to mean that they wanted to be a sorcerer who could be the arcane role in the party. Which they can do, even with going for a theme. It might look different, but there is no reason you can't do it.I read through that paragraph and I see requirements that fit the wizard's strengths, such as a "party arcanist" suite of spells, but I see nothing that focuses on what makes the sorcerer a sorcerer. There's a bit about the acid focus that says at least there's some toward the subclass, but everything about the general class is describing a wizard class, not a sorcerer class. It's no surprise it was a poor fit.
To me, the real knock for sorcerers (which is one of my favorite 5e classes to play and why it's frustrating) is their super lack of spells known. They are fluff supposed to be innately magical, yet both the Bard and Warlock who are also Charisma arcane casters, know more magic than default Sorcerers.
Spells known at the top end by Subclass (including UA):
- Baseline: 6 cantrips & 15 spells of any levels
- Aberrant Mind: base + 2 per spell level from 1st-5th (total 6 + 25)
- Divine Soul: base + 1 spell based on alignment affinity choice
- Draconic Bloodline: base + "mage armor" from Draconic Resilience
- Giant Soul: base + 1 cantrip, 1x 1st level, 1x 2nd level based on Giant Bloodline
- Shadow Magic: base + darkness
- Storm Sorcery: base (in UA it was base +2/spell level like Aberrant Mind)
- Wild Magic: base
- Bard baseline: 4 cantrips & 22 spells of any levels (4 of which can be from any spell list)
- Glamour: can do command once/long rest so kind of like a spell known
- Lore Bard: +2 spells from any class
- Warlock baseline: 4 cantrips & 15 spells of any level up to 5th + 1 each of 6th-9th for a total of 19 spells known
To me, it's odd and unreasonable that the most "pure" arcane caster knows the least amount of spells. Which is why I give them more spells known at my table!
As for a Black Dragon theme... well what are you trying to do @Sword of Spirit that you don't feel like you can get done?
In 5e dragons don't have any innate magic other than their breath weapon.
You could play a Black Dragonborn and start the game with a 30' line Acid line Breath Weapon. You could also take the XGE feats for Dragonborn to get a Fear Aura like the Dragon's Frightful Presence as well as claws and scaled armor (redundant with the Draconic Bloodline class feature). Not to mention Acid Resistance at 1st level.
Or forego the Dragon Hide feat (armor + claws) and wait to get Alter Self at 3rd level. 1 hour, extendable to 2 with the right metamagic. That gives you the ability to breath water and swim like a Black dragon, but also grow claws (when you're not underwater) or a bite to attack like a Black Dragon. You can swap between being amphibious to natural weapons as an action each turn for the duration and Quicken some other spell still.
So far I've only touched one the default 15 spells known.
If it's "acidy" things you want to do, there is Acid Splash (cantrip), Chromatic Orb (1st), Dragon's Breath (2nd), and Vitriolic Sphere (4th - like Fireball but lasts 2 rounds and one level higher). Plus you have an innate breath weapon as a Dragonborn.
As a Draconic Bloodline you're adding your Charisma to most of the damages on those as well so they only get better.
So using 4 of the 15 spells known +1 cantrip and a feat (at 4th), you've got 4 or 5 ways to deliver acid damage, claws/bite, swimming & water breathing, as well as a frightful presence effect.
Mix in Quicken spell to toss out Orbs & Splashes or Spheres and Splashes and you've got a good combat baseline.
Then you have 11 spells known and 5 cantrips worth to play with over the levels to get things like Flight (until you get your wings at 14th level) and whatever else you think you might want as a caster.
I know "fire" get all the spells, but tons of things have fire resistance too. Few have acid resistance, plus with the Orb and the Dragon's Breath, you can change to different energy types each combat/casting. Also you have room to add more damage types in cantrip form.
Sorry, this got long.